1. Field of the Invention
The present invention relates to a refined magnesium material such as an ingot or billet used for parts of transport and home electric apparatus as well as kinds of cases and a process for producing the refined magnesium material.
2. Description of the Prior Art
Aluminum alloy is widely used for a case such as an oil pan and a transmission case in a vehicle. Much attention has been paid to a magnesium alloy, because the parts can be made lighter from the magnesium alloy than an aluminum alloy. Further, a magnesium composite material having reinforcing agents added therein has been investigated. The present invention relates to magnesium, various magnesium alloy and magnesium composite, all of which hereinafter are referred to as "magnesium material".
In general, the magnesium material in a molten state is highly flammable when it comes to be in contact with air, and thereby, the molten magnesium material is more difficult to handle than the molten aluminum material.
(1) The die casting or the squeeze casting of magnesium material must be carried out under a condition wherein the molten magnesium material to be cast is separated from air by inflammable gas SF.sub.6 or a mixture of SF.sub.6 and CO.sub.2. On the other hand, the gravity casting must be carried out under a condition wherein the molten material to be cast is overspread by a flame resistant flux mainly containing sulfur. However, there are the following problems. The overspread gas, SF.sub.6 is expensive and results in a high manufacturing cost. The gravity casting generates SO.sub.2 gas due to the sulfur powder and results in a poor working environment.
(2) In a case of refining a returned magnesium material or scrap, there are the following problems. In order to prevent the molten magnesium material from catching fire, the refining process must be carried out by using a flux agent, which makes the manufacturing cost expensive and causes the resultant magnesium material to be inferior in corrosion resistance.
(3) The casting process of the magnesium material is not carried out in the way exactly the same as that of aluminum costing process in view of the facility and the working steps. When a die casting of a hot chamber type is applied to the magnesium material, a specified die casting machine is required. When the die casting of a cold chamber type is applied thereto, watching for the prevention of fire is required. This prevents the automatic casting of magnesium material. Further, it is difficult to apply a lost wax process to the casting of magnesium material.
These disadvantageous points result from the intrinsic property of magnesium material such as an easy flammability of the molten magnesium material, resulting in difficulty in ensuring safe operation and high-cost.
In order to solve the problem, one of inventors, Tadayoshi Nakamura has proposed a method for providing the molten magnesium material with a flame-resistant property by adding an alkaline earth metal or metals such as calcium to the molten magnesium material and further a method for recovering the original corrosion resistance of the magnesium material, which is degraded due to addition of alkaline earth metal, by adding a corrosion resistant metal such as zinc (Japanese Patent Application No. 54394/1992).
However, even if the magnesium material is provided with alkaline earth metal, it does not show sufficiently the flame resistant property, resulting in generation of some ignition point. Although the ignition point may self-extinguish, the ignition point may extend and develop to fire, so that the extinguishing agent SF.sub.6 must be used to put the fire out. On the other hand, when the molten magnesium material to which alkaline earth metals as a flame resistant agent are added is cooled and solidified into an ingot, the resultant ingot is always provided with porosity (which means hereinafter that a number of concave like points of less than 2 mm appeared on a cross-section of the cast body). It is a big problem because the porosity is hard to remove.
Accordingly, a first object of the present invention is to provide a method for producing the refined magnesium material which is more improved in the flame resistant property and is easy to handle and safe, that is, a method for suppressing generation of the ignition points.
A second object of the present invention is to provide a refined magnesium material which has the flame resistant property due to such an alkaline earth metal added thereto and includes substantially no porosity, which is easy to be generated when the molten magnesium material is cooled and solidified, if the flame resistant agents such as alkaline earth metals are added to the molten magnesium materials.
There is currently proposed a method for refining the magnesium alloy in Japanese Patent Publication 291350/1991 (unexamined), wherein a similar way to that of a refining method for aluminum material is applied to the molten magnesium material, that is, the magnesium material in a molten state is subjected to a bubbling process to adhere the impurity to the bubbles, resulting in floatation of the impurity on the surface of the molten magnesium material. In order to keep the impurity floating on the surface of the molten magnesium material, which might be otherwise dragged into the molten magnesium material due to the bubbling, it is necessary to blow up the bubbles quietly and uniformly in a way not to disturb the surface of the molten magnesium material. Further, in order to suppress the circulation movement in the molten magnesium material, which causes the impurity dragging, the upward movement speed of the bubble is devised higher than the downward movement speed in the molten magnesium material. Additionally, suppression of the oxidation at the surface of the molten magnesium material is attempted. It is not difficult not to disturb the surface of the molten magnesium material, but it is difficult practically to speed up the upward movement speed of the bubble without circulation movement as well as to prevent the oxidation on the surface of the molten magnesium material. Consequently, it is troublesome and causes loss of the efficiency in the refining process to satisfy all these restrictions.
In addition, among methods wherein the impurity can be removed, there is provided a refining method of returned material or scrap (secondary refining process). This method is to use a melting flux (Dow Chemicals), such as #230 or refining flux #310 including, as a main ingredient, potassium chloride or magnesium chloride. The melting flux is used to prevent the molten magnesium from catching fire, which requires the whole of the surface of the molten magnesium material to be covered by the flux. As a result, a part of the melting flux remains in the molten magnesium material, resulting in degradation of the mechanical property and the resistance to the corrosion of the resultant ingot of magnesium alloy.
Anyway, the conventional refining method hardly suppresses occurrence of the ignition points and the generation of the porosity in the solidified ingot of the magnesium material.